Process for producing oxygen



Dec. 25, 1951 JENNY 2,579,498

PROCESS FOR PRODUCING OXYGEN Filed DEC. 21, 1946 2 SP'IEETS-SPEET 1 2 ATT fEY Dec. 25, 1951 F. J. JENNY PROCESS FOR PRODUCING OXYGEN 2 SHEETSSI- EET 2 Filed Dec. 21 1946 L Y re 5 mm A M Patented Dec. 25, 1951 PROCESS FOR PRODUCING OXYGEN Frank J. Jenny, New York, N. Y., assignor to Hydrocarbon Research, Inc., New York, N. Y., a corporation of New Jersey Application December 21, 1946, Serial No. 717,672

16 Claims. 1

This invention relates to the production of oxygen by the liquefaction and rectification of air, and more particularly to an economical method of obtaining oxygen in high purity and in high yield without the use of chemical reagents to effect the removal of carbon dioxide present in air.

All temperatures herein are in degrees F. and pressures in pounds per square inch gauge.

Oxygen is commonly produced by partial liquefaction of air and rectification at low temperatures; preferably rectification is conducted in two stages at different pressures. The refrigeration necessary for liquefaction is supplied to the air, after it has been compressed and water-cooled to approximately room temperature. by indirect heat exchange with the effluent products of rectification. However, an additional amount of refrigeration must be supplied to compensate for cold losses resulting from the difference in enthalpy between the incoming air and the outgoing products of rectification and for heat leaks into the system. Methods of supplying this refrigeration heretofore used involve compressin at least a portion of incoming air to pressures as high as 3000 pounds and expanding with or without the performance of work to produce a temperature drop; or compressing all the incoming air to about 600 pounds and after the air has been partiall cooled by the products of rectification expanding a portion of the air. These methods are wasteful from the standpoint of compressor energy and require a great deal of equipment in the form of extra compressors, intercoolers and expanders.

For economical operation it is essential to recover the cold content of the outgoing products cf rectification. This is usually accomplished by passing these products in heat transfer relationship with the incoming air. In older syste s, in order to avoid deposition of frost and solid carbon dioxide in the tubular countercurrent heat exchange s through which the air is passed in indirect heat exchange relation with t e outgoing products of rectification, the air is trertod in driers and caustic scrubbers to remove water and carbon dioxide prior to admitlance of the .air into t e heat exchangers. Even with this treatment the exchangers had to be thawed out regularly to remove the frost (which tern is used in a generic sense to include both snow and ice) which caused stopping up of the ap aratus.

Mo e recently it has been suggested to use cold accumulators or regenerators (hereinafter referred to as heat exchangers) of large cold absorbing capacity through which the warm incoming air and the cold products of rectification are alternately passed with periodically reversed operation so that streams of warm air are flowed through the same packing-filled spaces that the cold separated oxygen and nitrogen traversed during the previous step in the process, the high boiling impurities deposited in these spaces during the passage of air therethrough being removed by sublimation during the subsequent flow in a reverse direction of the products of rectification. The use of these reversing heat exchangers in a process in which the air is compressed to relatively high pressure results in more costly operation from the standpoint of horsepower requirements because upon every reversal. which may take place ever three minutes, the volume of compressed air in the heat exchangers is lost and must be again replaced. Moreover, in the operation of such reversing heat exchangers it is important not to let the temperature at the exit end of the exchangers drop to a point where a part of the air becomes liquid because this liquid adheres to the surface of the exchangers and is wasted upon reversal of flow.

It is an object of the present invention to provide a process for producing oxygen by the liquefaction and rectification of air in which (1) carbon dioxide and preferably also moisture are removed from the air without the use of chemical reagents. (2) reversing heat exchangers are used through which flow in heat exchange relation the outgoing products of rectification and the incoming air, the process being operated at relatively low pressures of the order of about to about pounds so that the loss of compressed air in the exchangers upon each reversal is small, 3) the reversing exchangers are operated so that carbon dioxide and preferably also the moisture are substantially completely removed from the air during its flow therethrough during one step of the process and upon reversal the carbon dioxide and the frost, if any, deposited in the exchangers during the preceding step are substantially completely removed and this without liquefaction of air taking place in the exchangers and (4) the refrigeration necessary to compensate for cold losses resulting from the difference in enthalpy between the incoming air and the outgoing products of rectification and for heat leaks into the system is supplied by expanding a minor portion of the air and this without entailing any loss of the oxygen content of the air introduced into the process.

Another object of this invention is to provide a process for producing oxygen of high purity and in high yield from air with greatly reduced equipment and power costs as compared with existing procedures.

Other objects and advantages of this invention will be apparent from the following detailed description.

In accordance with this invention a stream of air is passed through a path in a heat exchange zone, a stream of rectification product is passed through another path in the heat exchange zone in heat exchange relation with the air passing therethrough, the air being thus cooled to a temperature close to its condensation point at the pressure existing in the heat exchange zone so as to substantially completely remove all carbon dioxide present in the air. With the substantially complete removal of carbon dioxide from the air stream, the expander may be operated without encountering the troubles heretofore experienced because carbon dioxide was solidified in the expander during the expansion of the air stream. A minor portion of the thus cooled air stream is thereafter warmed by heat exchange with one of the warmer streams of fluid media fiowing in the system, preferably by passage through the aforesaid heat exchang zone, to a temperature such that upon subsequent expansion little or no condensation or formation of liquid air in the expander takes place with consequent further improvement in the operation of the expander. The thus warmed minor portion is expanded to produce the refrigeration necessary to compensate for cold losses resulting from the difference in enthalpy between the incoming air and the outgoing products of rectification and for heat leaks into the system, and the expanded air is introduced into the rectification system where it is rectified to recover the oxygen content thereof. The remaining major portion of the air is also introduced into the rectification system where it is rectified to recover the oxygen content thereof. The flow of air and that of the rectification product are periodically reversed through their respective paths in the heat exchange zone so that upon each of the reversals the rectification product substantially completely removes the carbon dioxide deposited during the preceding step of the process.

More specifically, in accordance with this invention, a stream of air at about 60 to about 100 pounds and a temperature of about 70 to about 110 F. is passed through a path in a heat exchanger containing at least three fiow paths in heat exchange relation with each other through one of which passes a stream of oxygen or nitrogen product of rectification. The stream of air leaving the first-mentioned path and cooled to a temperature below about 270 F. but above its condensation point, is divided into two streams, one comprising the major portion of the air flowing to the rectification system, and the other comprising the remaining minor portion, say from 15% to 35% by volume of the total air introduced into the process. being warmed by heat exchange with a warmer stream in the process to a temperature such that little or no liquid air is formed upon subsequent expansion of the warmed air stream. The thus warmed air stream is expanded to produce the refrigeration necessary to compensate for cold losses resulting from the difference in enthalpy between the incoming air and the outgoing products of rectification and for heat leaks into the system. The

expanded air is introduced into the low pressure stage of the rectification system where its oxygen content and nitrogen also. if desired, are recovered. The low pressure stage is preferably operated under a pressure of from about 4 to about 10 pounds, preferably at about 5 pounds; the high pressure stage is maintained at a pressure of from about 60 to about pounds.

At the colder end of the exchanger, where the oxygen or nitrogen products of rectification enter and air leaves, there is maintained between the rectification product stream and the countercurrent stream of air a temperature difference in the range of from about 5 to about 10 F'., preferably about 6 to about 8 F. This is accomplished by passing the cold rectification product stream in heat exchange relation with a warmer stream in the system thereby conservin the refrigeration in the rectification product stream and warming this stream to a temperature of from about 5 to about 10 F. below the temperature at which the air stream leaves the exchanger. The warmed rectification product stream at this temperature is introduced into its flow path in the exchanger. Periodically the flow of air and that of nitrogen or oxygen through their respective paths are reversed so that upon reversal the air flows through the path through which during the preceding step the nitrogen or oxygen had passed and the nitrogen or oxygen flows through the path through which had previously passed the air. The nitrogen or oxygen removes, by sublimation or evaporation, the carbon dioxide and the frost, if any, deposited in the exchanger during the preceding step.

Operating in this manner substantially complete purging of carbon dioxide is obtained upon each reversal of fiow. Likewise complete purging of deposited frost, if any, is obtained so that the equipment may be operated continuously.

In the accompanying drawings forming a part of this specification and showing, for purposes of exemplification, preferred layouts of equipment for practicing the process of this invention,

Figure 1 illustrates diagrammatically a preferred layout of apparatus for practicing the process of this invention;

Figure 2 illustrates a modified arrangement of heat exchangers which may be used in lieu of the exchangers shown in Figure 1;

Figure 3 illustrates a modified arrangement of apparatus for practicing the process of this invention, and

Figure 4 illustrates still another modified arrangement of heat exchangers which may be used in lieu of the exchanger shown in Figure 3.

It will be understood the drawings illustrate diagrammatically preferred apparatus for practicing this invention and that the invention may be carried out in other apparatus. For example, any desired number of reversing exchangers may be used in lieu of the reversing exchangers shown in the drawings: each of the reversing exchangers of Figures 1 to 4 inclusive may be replaced by two or more smaller exchangers placed in series and/or parallel, if desired; other rectification systems may be used in lieu of those shown in Figures 1 and 3 including rectification systems equipped with means for purging the high pressure stage to remove incondensible gases such as helium, hydrogen and neon therefrom, or the rectification system of Figure 1 may be used with the exchangers of Figures 3 and 4 or the rectification system of Figure 3 may be used with the exchangers of Fi ures 1 and 2.

In the drawings like reference characters indicate like parts.

Referring to Figure 1, I is a heat exchanger which may be of any well-known type. In the embodiment shown in the drawings it consists of a single shell in which are provided three flow paths, namely, interior path II and concentric paths i2 and I3 disposed in heat exchange relation with each other. The heat exchanger has in each of the paths suitable fins of heat conducting material. e.g., copper or aluminum, promoting rapid and eflicient heat exchange between the gaseous media flowing therethrough. For purposes of illustration and in the interests of simplicity. each flow path in an exchanger is shown on the drawings as consisting of a single tube, the several paths being disposed concentrically. Actually, however, each path in each exchanger may comprise a multiplicity of tubes for flow therethrough. One form of exchanger which may be used in practicing the process of this invention is disclosed and claimed in application Serial No. 676,142, filed June 12, 1946. As the construction of the heat exchanger per se does not form a part of this invention and as it may be of any well-known type it is believed further description thereof is unnecessary.

Paths I2 and I: are the paths through which air and nitrogen flow. the flows of these two media through their respective paths being periodically reversed so that during one step of the process air flows through path l2 and nitrogen through path l3. and upon reversal during the suceeding step air flows through path I3 and nitrogen through path l2. Reversal of flow is accomplished by suitably positioning the compound reversing valves l4 and it which may be of any well-known type. Valve I4 is disposed in the pipe line system consisting 01 (a) air inlet pipe I5 leading into valve II, (b) nitrogen exit line H leading to any suitable point of nitrogen disposal and (c) pipe lines l3 and i3 leading to one end of paths i2 and I3. respectively. Lines and 2| lead from the other end of the exchanger from paths l2 and i3. respectively, to valve IS. A line 22 leads from valve I5 to an air line 23, portion 24 of which leads into a line 25 communicating with path II. A valve 26 in line 25 controls how therethrough.

By suitably positioning valve 25, a portion of the total air which is to be expanded. say from about 10% to 100% by volume, preferably about 10% to 35%, flows through line 25 into and through path II where the air is warmed, imparting most of its cold content to the air flowing in a countercurrent direction. The thus warmed air leaves path ll through line 21 communicating with a pipe line 23 leading into an expander 29 which may be a centrifugal expander or turbine of any well known type. Leading into line 28 is an air line 3|). flow through which is controlled by valve 3| and which communicates with air line 23. By suitably positioning valve 3|, the remaining portion of the total air to be expanded, say from about 90% to 0% by volume, preferably about 90% to 65%, flows from line 23 through line 30 into line 28 where it mixes with the warmed air entering line 28 from line 21. the resultant air mixture thus being warmed to a temperature such that no condensation or formation of liquid air takes place in expander 23 with consequent improvement in the eiilciency of the expander operation. The expanded air leaves expander 23 through line 32 leading into the low pressure stage of a rectification system hereinafter described.

A second heat exchanger 33 is provided in the form of a shell having therein flow paths 34. 33 and 36 each provided with fins to promote heat exchange as in the case of the exchanger Iii. Path 35 is the path through which a minor portion of the nitrogen product of rectification flows from the rectification system hereinafter described by way of a pipe line 31 communicat ing with path 36. Flow through line 31 is controlled by valve 33. The nitrogen product of rectification leaves path 36 through line 33 which leads into the nitrogen line 40 communicating with valve l5 associated, with the heat exchanger l3.

Oxygen and air periodically flow through paths 34 and 33 in heat exchange relation with each other and with the nitrogen flowing through path 33. The flows of air and oxygen through their respective paths are periodically reversed so that during one step of the process air flows through path 34 and oxygen through path 35 and upon reversal during the succeeding step air flows through path 35 and oxygen through path 34.

Reversal of how is accomplished by suitably positioning the compound reversing valves 4| and 42 which may be of the same type as valves I4 and I5. Valve 4| communicates with the main air line through a pipe line 43 and is connected by lines 44 and 45, respectively, to the warm end of the paths 34 and 35. Valve 4| is provided with an oxygen exit line 45' leading to a suitable oxygen storage tank or point of consumption. Valve 42 is connected to the cold end of paths 34 and 35 by lines 48 and 41. respectively. Air line 43 leads from valve 42 to the main air line 23. Oxygen is supplied to valve 42 through the line 43 which leads from the rectification system hereinafter described.

In the reversing exchangers Ill and 33 shown in Figure 1, a minor portion of the air from line 23 flows through path H in heat exchanger III in heat exchange relation with air and nitrogen flowing through the other two flow paths l2 and I3 in this exchanger, while a minor portion of the nitrogen flows through path 36 in heat exchanger 33 in heat exchange relationship with the oxygen and air flowing through the flow paths 34. 35 and this minor nitrogen stream is then mixed with the remainder of the nitrogen to form the nitrogen stream flowing into and through line 40. It will be understood that it desired the minor nitrogen stream may be passed through flow path II in exchanger It in heat exchange relation with the air and nitrogen streams fiowing through the other two paths in this exchanger and the minor air stream passed through flow path 33 in heat exchanger 33 in heat exchange relation with the air and oxygen passing through the other two flow paths in this exchanger.

Reversing exchangers l3 and 33 may be placed in vertical, horizontal or any other desired position. When these exchangers are arranged vertically the colder end may be above or below the warmer end. While two exchangers have been shown, it will be understood that any desired number may be employed. In general, the nitroren flow paths through the exchangers should have approximately four times the volumetric capacity of the oxygen flow paths. If desired. exchangers in which the oxygen and nitrogen flow paths are of the same volumetric capacity may be employed in which case tour air-nitrogen rcversing exchangers are employed for each airoxygen reversing exchanger. Also, of the total air cooled by indirect heat exchange with the oxygen and nitrogen products of rectification, about 20% flows through the air-oxygen reversing exchanger and about 80% through the air-nitrogen exchanger.

The rectiflcation system 58 comprises two columns and 52. Column 5| is operated at a pressure of from about 60 to about 100 pounds, preferably at about 70 to 75 pounds and column 52 at a pressure of from about 4 pounds to about pounds, preferably at about 5 pounds. These columns, as customary, are provided with rectiflcation plates of the bubble-cap or other desired type. Air is supplied to the base portion of the high pressure column 5| through a line 53 which leads from line 23 and passes through non-reversing heat exchangers 54 and 55 hereinafter more fully described. Crude oxygen containing approximately 40% oxygen, the rest being chiefly nitrogen, flows from the base of column 5| through line 56 which passes through a nonreversing heat exchanger 51. Upon flow through the expansion valve 58 in line 56 the crude oxygen is flashed entering column 52 at 58. A line 63 leads from the top of column 5|, passes through a non-reversing heat exchanger 5| into a line having one branch 62 for returning liquid reflux comprising chiefly nitrogen to column 5| and another branch 63 passing through a non-reversing exchanger 64 and leading into the low pressure column 52 at 65. An expansion valve 65 is disposed in branch 63.

As hereinabove described, expanded air from expander 29 enters the low pressure column 52 through line 32. The base of this column is provided with a line 61 passing through the nonreversing heat exchanger 6|, this line having a return portion 58 leading into the low pressure column 52 at 69. The lines 61 and 68 and the cooperating heat exchanger 5| function as a reboiler; liquid oxygen flows through line 81 through exchanger 6| in indirect heat exchange relation with the gaseous stream comprising chiefly nitrogen passing through line 60 which causes vaporization of the liquid oxygen to take place, the oxygen vapors flowing into column 52 at 53. Nitrogen line 10 leads from the top of column 52 through exchangers 64, 51 and 55 and communicates with line 40 leading to the valve l5 of reversing heat exchanger Ill and line 31 which communicates with the flow path 35 in reversing heat exchanger 33.

In operation of the equipment of Figure 1 air from the main air line is divided into two streams, one of which flows through line I6, valve |4, line I8 through flow path l2, leaving through line and flowing through valve |5, line 22 into line 23. The air stream is thus cooled to a temperature near its condensation point and all carbon dioxide and moisture, if any, removed therefrom and deposited in the flow path l2. A portion of this air stream flows through line 24, valve 25, line 25 through flow path II where it is warmed by heat exchange with the countercurrent air stream. The warm air stream leaves this flow path through line 21 and flows into line 28. Another portion flows from line 23 through valve 3|, line into line 28 where it mixes with the warmed air from line 21, the mixture entering expander 28 at a temperature such that substantially no liquid air is formed in the expander 23. The expanded air from expander 29 flows through line 32 and enters the low pressure stage of the rectification system.

Nitrogen flows from the low pressure column 52 of the rectification system through line 13, a minor portion of this nitrogen stream, say from about 2% to 6% by volume flows through valve 38, line 31, flow path 36 in exchanger 33 through line 33 into line 40, the nitrogen thus being warmed by heat exchange with the air stream flowing through either path 34 or 35 in reversing exchanger 33. The thus warmed nitrogen stream mixes with the major portion of the nitrogen flowing from line 10 through line 40 and the mixture enters valve |5 and flows from this valve through line 2| into and through flow path I3 in reversing exchanger Ill. The nitrogen leaves flow path I3 through line I! and flows through valve l4 into the nitrogen exit line H.

Another air stream flows from the main air line through line 43, valve 4|, line 44 into and through flow path 34 of reversing exchanger 33. The air cooled to a temperature close to its con densation point leaves flow path 34 through line 46 and enters valve 42 leaving this valve through line 48 which communicates with the air line 23. From line 23 a minor portion of the air as hereinabove described flows through line 30. The remaining major portion flows through line 53 into non-reversing exchanger 54 where the air is further cooled by the oxygen flowing in indirect heat exchange relation therewith through line 49. From exchanger 54 the air flows through exchanger 55 where it is still further cooled by flowing in indirect heat exchange relation with the nitrogen passing through this exchanger, the thus cooled air entering the high pressure column 5|. Simultaneously oxygen from the low pressure column flows through line 49, exchanger 54, valve 42, line 41, flow path 35 where the oxygen flows in heat exchange relation with the air and nitrogen flowing through paths 34 and 36, respectively. The oxygen leaves flow path 35 through line 45 and flows through valve 4| and thence into the oxygen exit line 45'.

Upon reversal as indicated by the dotted setting of reversing valves I4, |5, 4| and 42, which reversal may take place every three minutes, air flows through line |5, valve |4, line l9 into flow path I3 of the heat exchanger ill and another air stream flows through line 43, valve 4|, line 45 into flow path 35 of heat exchanger 33. The air cooled to a temperature close to its condensation point leaves heat exchanger I0 through line 2|, flows through valve |5, line 22 into line 23. A minor portion of the air from this line flows through line 24, valve 26, line 25, flow path H in heat exchanger III, the air leaving this flow path through line 2! and flowing into line 28 which communicates with the expander 29. The air leaving flow path 35 of heat exchanger 33 during this step of the operation flows through line 41, valve 42, line 48 into line 23. From line 23, as in the previous step of the process, a minor portion of the air flows through valve 3|, line 30, entering line 28 where it mixes with the air flowing through this line from line 21. The expanded air leaves expander 29 through line 32 and enters the low pressure column 52. Simultaneousiv, a minor portion oi the nitrogen flowing through line 10 which has passed through exchanger 55 flows through valve 38, line 31, flow path 36 through line 39 into line 40. The remaining major portion of the nitrogen stream from line 10 flows through line 40 mixing with the warm nitrogen stream entering this line from line 33 and the resulting mixed ntirogen stream passes from line an through valve I 5, line 28, flow path l2, leaving this flow path through line I3 and passing through valve l4 into the nitrogen exit line I1. Oxygen flows through line 43 into valve 42 through line 4!, flow path 34 in exchanger 33, through line 44, valve 4! and exits through the oxygen exit line 45'.

Thus it will be noted that flow throu h the flow paths II and 36 of the exchangers l and 33, respectively, always takes place in the same direction throughout the operation of the process. On the other hand, flow or the nitrogen and air through their respective flow paths in exchanger IB is periodically reversed and the flow of the oxygen and air through their respective flow paths in exchanger 33 is periodically reversed.

The air entering high pressure column is rectified, crude oxygen is withdrawn from the base of this column through line 55, chilled by flowing in indirect heat exchange relation with the nitrogen stream in exchanger 51, flashed by flowing through expansion valve 59 so that it is still further cooled and introduced into the low pressure column 52 at 58. The gaseous stream consisting chiefly of nitrogen flowing through line 03 of the high pressure column 5| passes through heat exchanger Si in indirect heat exchange relation with the oxygen flowing through this exchanger, this gaseous stream being thus cooled and entering lines 62 and 63. The gaseous stream is thus substantially condensed, liquid flowing through line 82 into column 5| where it serves as reflux liquid. The remainder of the stream predominating in nitrogen flows through line 83, non-reversing exchanger 64 where it is cooled by the nitrogen flowing through this ex- 1 changer, then through the expansion valve 66 where it is flashed thus further cooling it and enters the low pressure column 52 at 65 as a vapor-liquid mixture in which the liquid predominates. Nitrogen leaves the top of column 52 through line "I, flows through heat exchanger '4 where it gives up a portion or its cold content as hereinabove described to the stream passing through this exchanger, and then through the heat exchanger 51 where it cools the crude oxygen stream flowing through this exchanger. From exchanger 51 the nitrogen stream flows through exchanger 55 where it cools the air stream flowing therethrough. From the exchanger 55 a minor portion 01' the nitrogen stream flows through valve 38 and the remaining major portion through line 40 as hereinabove described. Oxygen from the base of column 52 flows through line 51 and exchanger 6| which functions as a reboiler, the resulting oxygen vapor entering column 52 at 83. The'product oxygen stream flows through line 43 leading from the lower portion of column 52 through exchanger 54 and valve 42 into one or the other of flow paths 34 and 35 of reversing exchanger 33 depending upon the setting of reversing valve 42.

The modification of Figure 2 differs from that of Figure l chiefly in the following respects:

(1) The exchangers Ill and 33 in the modification of Figure 2 are divided into two sections, IIA. MB and 33A, 3313 respectively;

(2) All of the air passed to the expander 29 may be warmed by passage through section iIlB instead of dividing a minor portion of the chilled air to be expanded into two streams one of which passes through flow path H in exchanger Ill and is thus warmed and then mixed with the other stream flowing through line to, the mixture being introduced through line 28 into the expander 29 as in the modification of Figure l; and

(3) A minor portion of the nitrogen stream is passed through section 333 only 01 exchanger 33 before mixing with the major portion of the nitrogen stream flowing through exchanger III; in the modification of Figure l a minor portion of the nitrogen stream flows through flow path 36 which extends the full length of exchanger 33.

In Figure 2 section IDA is provided with two reversing flow paths HA and HA for flow of air and nitrogen therethrough. Section NIB is provided with reversing flow paths I23 and HE for flow of air and nitrogen therethrough. Paths HA and I2B are connected by a line H and flow paths ISA and I3B by line I2. Section IIIB also contains a flow path IIB for flow of a minor portion of the cooled air stream therethrough, this flow path leading into a line 13 which connects the exit end of the flow path to expander 28.

Section 33A is provided with two reversing flow paths 34A and 35A for flow of air and oxygen therethrough and section 33B is provided with flow paths 34B and 358 for flow of air and oxygen therethrough. Flow path 34A is connected with flow path 348 by a line I4 and flow path 35A with 353 by a line 15. Section 33B also contains the flow path 3GB for flow of a minor portion of nitrogen therethrough, the exit end of this flow path communicating with a line 15 which leads to the nitrogen line 40. Each of the flow paths hereinabove described have suitable walls and flns of heat conducting material, e. g., copper or aluminum, promoting rapid and efficient heat exchange between the gaseous media flowing therethrough.

The line 48 leading from valve 42 is provided with a line 48'. flow through which is controlled by valve 48" for introducing, if desired, some of the cold air flowing through line 48 into the warm air stream flowing through line 13 into the expander 29. thereby permitting more accurate control of the temperature of the air entering expander 29. Line 48' corresponds to line 33 of Figure 1 except that line 48 leads from line 48 and not from the air line 23 as in Figure 1. I! desired. line 48 may lead from line 23 as is the case with line 30 of Figure l.

The remaining parts of Figure 2 are the same in structure and function as those shown in Figure 1, have been given like reference numerals and it is believed the structure and operation of these parts will be clear from the above description of the like parts in Figure 1.

In the operation of the exchanger system of Figure 2 air from the main air line flows into line l6, valve 14, line l8, flow path I2A, line H, flow path I2B, line 20, valve l5, line 22 into line 23. A minor portion of the air from line 23 flows through line 24, valve 26, line 25 into flow path IIB. From this path the warmed air flows through line 13 into expander 29, the expanded air flowing through line 32 into the low pressure stage of the rectification system. Another stream of air flows from the main air line through line 43, valve 4|, line 44, flow path 34A, line i4, flow path 343, line 46, valve 42, line 48 into line 53 where it mixes with the air from line 23. Line 53 leads to the heat exchangers 54 and 55 from the latter of which the air passes to the high pressure stage of the rectification system.

A minor portion of the nitrogen stream passing through line 10 flows through the valve 38 in line 31, enters and flows through flow path 353 where the nitrogen is warmed, the resulting warmed nitrogen stream flowing through line 19 into line 49 where it mixes with the remainder of the nitrogen stream entering line 49 from line 19. The thus warmed mixed nitrogen stream flows through valve I5, line 2|, flow path I3B. line 12, flow path I9A, line I9, valve I4 into the nitrogen exit line I1. Oxygen from line 49 leading from the low pressure stage of the rectification system passes through heat exchanger 54 into valve 42, line 41, flow path 353, line 15, flow path 35A, line 45, valve 4I into the oxygen exit line 45'.

Upon reversal as indicated by the dotted setting of reversing valves I4, I5, 4| and 42, air flows through line I6, valve I4, line I9, flow path I3A, line 12, flow path I3B, line 2|, valve I5 and line 22 into line 23. A minor portion of the air in line 23 flows through line 24, valve 29, line 25, flow path HE and line 13 into the expander 29. Another air stream flows through line 43, valve 4|, line 45, flow path 35A, line 15, flow path 35B, line 41, valve 42 and line 48 into line 53 leading to the high pressure stage of the rectification system. A minor portion of the nitrogen stream flows through line 31, flow path 353, line 16 into line 49 where it mixes with the major portion of the nitrogen stream, the resulting mixture flowing through valve I5, line 29, flows path I2B. line 1|, flow path I2A, line I9, valve I4 and the nitrogen exit line I1. Oxygen flows through line 49, valve 42, line 49, how path 34B, line 14, flow path 34A, line 44 and valve 4| to the oxygen exit line 45'.

It will be understood that if desired an exchanger such as 33 of Figure l in which the nitrogen stream flows through the entire length of the exchanger may be used with a sectional exchanger involving sections WA and I9B as shown in Figure 2 through which the air stream to be warmed passes through only one of the sections of the exchanger. Also a sectional exchanger corresponding with sections 33A and 33B of Figure 2 may be used with an exchanger such as exchanger I9 of Figure l in which the air stream to be warmed passes through a flow path extending the full length of the exchanger.

In the modification of Figure 3, exchanger 89 is employed, provided with flow paths BI and 82 for flow of air and nitrogen, the flows of these two mediums being periodically reversed through their respective flow paths. A flow path 83 in heat exchange relation with the other flow paths is provided for uni-directional flow of oxygen. A flow path 84 is provided for uni-directional flow of a minor portion of the chilled air stream leaving the exchanger to effect reheating of this air stream prior to its introduction into the expander 29. A flow path 85 is provided for unidirectional flow of a minor portion of the nitro gen stream, the warm nitrogen stream leaving flow path 95 as hereinafter more fully described, mixing with a major portion of the nitrogen stream to elevate the temperature of the mixed nitrogen stream which is passed through flow path 9| or 82, so that the mixed nitrogen enters at a temperature within 5 to 19 F., preferably 6 to 8 F., of the exiting air stream.

As in the other modifications. each of the flow paths is provided with suitable walls and tins of heat conducting material, e. g., copper or aluminum. The various flow paths, it will be noted from Figure 3, are arranged in concentric relationship.

The main air line leads to a valve I4 disposed in the pipe line system consisting of a nitrogen exit line I1, a line I9 communicating with the warm end of flow path 9| and a line I9 communicating with the warm end flow path 82, this valve and associated lines being the same as the corresponding parts of Figure l and hence have been given like reference characters. The cold ends of flow paths 9| and 92 communicate with line 29 and 2|, respectively, which lead into a valve I5. Line 22 leads from valve I5 into a line 95, one branch of which leads to the rectification system and another branch 91 of which is provided with a valve 99 and leads to the inlet of the flow path 94. A branch line 99 leads from line to a line 99, leading into the expander 29. Flow through branch line 99 is controlled by valve 9|. Of the cold air flowing through line 22 from valve I5, a minor portion, say about 2%, flows through line 81, valve 99, flow path 94 into line 92 leading into line 99. Another minor portion, say about 18%, flows through valve 9|, line 99 into line 99 where it mixes with the warm air from line 92, the air stream entering expander 29 being thus warmed to a temperature such that no formation of liquid air within the expander takes place. From the expander 29 the expanded air flows through line 32 to the low pressure stage of the rectification system hereinafter described.

The rectification system of Figure 3 comprises a two-stage rectification column 93, the lower section 94 of which is operated at a pressure of about 69 pounds to about 100 pounds, preferably about '70 to 75 pounds, and the upper section of which is operated at a pressure of from about 4 pounds to about 10 pounds, preferably at about 5 pounds. This column, as is customary, is provided with rectification plates of the bubble-cap or other desired type. The lower section 94 communicates with a condenser 95 and has a liquid collecting shelf 91 disposed immediately below the condenser 96 for collecting liquid nitrogen. Pipe line 99 leads from this shelf 91 to a non-reversing heat exchanger 99 which in turn communicates through pressure reducing valve I99 with the top portion of the upper section 95. Condenser 96 acts as a reboiler for the upper section 95.

From the base portion of lower section 94 a pipe 1ine I9| for the flow of crude oxygen passes to a non-reversing heat exchanger I92 which communicates through pipe line I93 having a pressure reducing valve I94 therein with the low pressure section 95 at an intermediate point I95. A line I96 having a pressure reducing valve I91 therein leads from condenser 95 to nitrogen line I98 leading to the non-reversing heat exchanger I99 through which the nitrogen passes in indirect heat exchange relation with the air stream flowing through line 86 to the high pressure section 94. Line |I9 leads from the top of low pressure section 95 to heat exchanger 99, the nitrogen flowing through this line passing through the heat exchanger 99, then through line III to heat exchanger I92, and then through line 2 which communicates with line I99. An oxygen line II3 leads from the lower part of low pressure section 95 to one end of flow path 93, the other end of this flow path being provided with an oxygen exit line II4.

A line I I5 leads from exchanger I99 for flow of notrogen therethrough. Line I I5 is provided with a branch I I6, flow through which is controlled by valve II1. Branch line IIB leads into one end of flow path 95, the other end being provided with an exit line III! which leads into the nitrogen line I|9 into which also flows the major portion 75 of the nitrogen stream from line I I5. A valve I29 I3 may be disposed in the portion 01 line Ill leading into line I I9.

It will be understood that two separate fractionatlng columns as shown in Figure 1 may be used with the exchanger system of Figure 3 in lieu of the two-stage column 93 and conversely a two-stage column may be used with the heat exchange system of Figure 1 in lieu of the two separate fractionating columns shown in that figure. It will be further understood that the equipment throughout is heat insulated to minimize loss 01 cold.

In the operation of the apparatus of Figure 3, air which is admitted from the main air line flows through valve II, line I9. flow path BI, line 20, valve l5, line 22 Into line 86. A minor portion of the air stream flows through line 91, valve 99 and flow path 84 into line 92. Another minor portion flows from line 96 through valve SI and line 99 into line 90 where it mixes with the warmed air from line 92, the thus warmed air stream entering expander 29 and the expanded air flowing through line 32 to the low pressure stage 95 of the rectification system. The major portion of the air flows through line 86 and heat exchanger III! where it is further cooled by nitrogen flowing in indirect heat exchange relation with the air, the thus cooled air stream entering the high pressure section 94 of the rectification column.

Nitrogen flowing through line I98 from lines I96 and H2 passes through the heat exchanger I99 as hereinabove described. A minor portion 01 this nitrogen stream flows through valve Ill and line IIG into flow path 85 where it is warmed by heat exchange with the gaseous mediums passing through the other how paths in exchanger 80, the warm nitrogen leaving through line H9 and flowing into line II9 where it mixes with the major portion of the nitrogen stream entering line 9 from line H5. The resulting mixed nitrogen stream passes through valve I5, line 2|, flow path 82, line I9, valve I4 and exits through the nitrogen exit line H. Simultaneously, oxygen flows from the low pressure section 95 through line H3, flow path 83 and leaves through the oxygen exit line I I4.

Upon reversal, no change in the direction of flow of the oxygen through flow path 83, of the minor portion of the air stream through flow path 84 and of the minor portion of the nitrogen stream through flow path 85 takes place, i. e., these streams flow in the same direction. Air, on the other hand, as indicated by the dotted setting of reversing valves I4 and I5, flows from the main air line through valve II, line I9, flow path 82, line 2|, valve I5 and line 22 into line 89, having one branch 81 leading to flow path 94, another branch 99 leading to line 90 and the main line 99 leading to the high pressure stage 94 of the rectification column 93. The mixed nitrogen stream produced in line ll9 by nitrogen flowing thereinto from lines H8 and IIS flows through valve 15, then through line 29, flow path BI, line III. valve I4, and exits through the nitrogen exit line II.

The exchanger modification of Figure 4 differs from that of Figure 3 chiefly in that the exchanger I26 has four flow paths therethrough which are identified by the same reference characters as those used to indicate the like flow paths of Figure 3; the flow path 85 for flow of a minor portion of nitrogen therethrough to be warmed prior to mixing with the major portion oi the nitrogen rectification product is not employed in the modification of Figure 4. In-- 14 stead a separate non-reversing exchanger I29. which exchanger may be of any well known type in which efllcient cold exchange takes place, is used.

In Figure 4 a line I21 having valve I22 therein leads from the nitrogen line I I5 to an exchanger I26. A line I29 leads from this exchanger to the nitrogen line II9 leading to valve I5. Line 92 leading from the warm end of flow path ll passes through exchanger I26, the warm air flowing through line 92 giving up a portion of its heat to the minor portion of the nitrogen passing through exchanger I26 and thus warming this nitrogen stream to a temperature such that. upon mixing with the remaining major portion of the nitrogen, the resulting mixture has a temperature of from 5 to 10 F. below that oi the air stream exiting from the colder end of the exchanger I25. From the exchanger I29 the air stream containing a substantial portion of the heat picked up in its flow through exchanger I25 enters line 99 where it mixes with the air stream flowing into this line from line 99. The resultant air mixture at a temperature such that no liquid air is formed upon expansion thereof in expander 29 is expanded in this expander and the expanded air passed through line 32 into the low pressure stage of the rectification system.

The remaining parts of Figure 4 are the same as the like parts of Figure 3 and have been given like reference characters. The construction and function of these remaining parts and the operation of the exchanger I25 should be clear to one skilled in the art from the above description.

The reversing exchanger of Figure 4 has the advantage over that of Figure 3 that it is less costly due to the elimination of one of the flow paths in the reversing exchanger; as a practical matter it has been found less expensive to emloy a separate exchanger I26 than to build a reversing exchanger having an additional flow path corresponding to flow path of Figure 3.

In the modification of Figure 4, instead of having a minor portion of the air flow through flow path 84 and the thus heated air passed through exchanger I26 where it gives up a portion of its heat to the minor portion of the nitrogen flowing through exchanger I26, the minor portion oi the nitrogen may be passed through flow path 94 and the thus warmed nitrogen passed through exchanger I26 in indirect heat exchange relation with an air stream passing to expander 29. the air thus being heated to a temperature such that no liquid air forms in expander 29 upon expansion 0! the warmed air stream. In such case, the nitrogen stream leaves exchanger I29 at a temperature such that when mixed with the remainder of the nitrogen product of rectification the mixture has a temperature within 5 to 10 F. below that of the exiting air stream at the colder end of exchanger I25.

One example of the operation oi the process 01 this invention in the apparatus shown in Figure 1 is described below. It will be understood this example is given for purposes of exempliilcation only and the invention is not limited thereto. The example refers to an oxygen plant operating in a locality where the atmospheric pressure is 13.1 pounds per square inch absolute.

Air under pressure of about 74.6 Pounds and at a temperature 01' 95 F. is supplied through line I9, valve I4, line III to the flow path I2 and through line 43, valve II, line H to the flow path 34' of heat exchangers III and 23. respectively. The air leaves flow path I2 through line 25, valve l and enters line 22 flowing into line 23: the other air stream leaves flow path 34 through line 46, valve 42 and flows through line 43 into line 23. The air leaves the flow paths I2 and 34 at a temperature of 275 F.

Approximately 20% oi! the total air introduced into the process is expanded in expander 23. Of this fraction which is to be expanded, approximately 12% flows through line 24 at a temperature of -275 F., entering and flowing through flow path I I, the air being thus heated to a temperature of 823 F.; the remaining 88% at a temperature of 275 F. passes from line 23 through line 35, producing in line 28 a mixed air stream having a temperature of 233 F. and a pressure of 72.9 pounds. In expander 28 this mixed air is expanded to a pressure of about 6.4 pounds, its temperature thus being reduced to -306 F., at which temperature and pressure it is introduced into the low pressure column 52.

The remainder of the air (approximately 80%) at a temperature of 275 F. flows through line 53 into heat exchanger 54 where it passes in heat exchange relation with an oxygen stream at a temperature of -293 F., flowing through line 43 from the low pressure column 52. The air is thus cooled to a temperature of 276.5 and the oxygen stream warmed to a temperature of -283 F. at which temperature it enters flow path 35 of exchanger 33. The air at a temperature of -276.5 F. passes through the heat exchanger 55 in heat exchange relation with the nitrogen stream flowing through line 1|) at a temperature of 293 F. and undergoes partial liquefaction (about 1.5% being liquefied) and passes at a temperature of 276.5 and a pressure of about 72 pounds into the high pressure column 5|. The nitrogen flowing through heat exchanger 55 is thus warmed to a temperature of -290.4 F.

About 2% by volume of the nitrogen stream at this temperature of 290.4 flows through line 31 and flow path 35. leaving this flow path at a temperature of 827 F., at which temperature it mixes with the remaining major portion of the nitrogen stream flowing through line 40. The resulting nitrogen stream has a temperature of 283 F., at which temperature it passes through valve l5, line 2| into heat exchanger HI. Thus the temperature differential between the entering nitrogen stream and the exiting air stream at the colder end of exchanger H! is 8 F. The nitrogen leaves flow path H! at a temperature of 82.7" F. and at substantially atmospheric pressure.

Oxygen at a temperature of 293 F. and a pressure of 6.6 pounds flows through heat exchanger 54 where. as above described, its temperature is elevated to 283 F., at which temperature it enters flow path 35 in heat exchanger 33, leaving this flow path at a temperature of 82.7 F. and at substantially atmospheric pressure. Thus the temperature differential between the entering oxygen stream and the exiting air stream at the colder end of exchanger 33 is 8 F.

In the operation of the rectification system 50, crude oxygen at a temperature of -280 F. and a pressure of 72 pounds passes through line 56 through heat exchanger 51 in indirect heat exchange relation with nitrogen flowing through line Ill. The crude oxygen is thus cooled to a temperature of -287.6 F. The crude oxygen is then flashed in its flow through expansion valve 59 entering column 52 as a vapor-liquid mixture at a, temperature of about 302.5 F.

and a pressure of about 6.2 pounds. Nitrogen at a temperature of 316.5 F. and a pressure of about 5.4 pounds flows through line 10 into heat exchanger 54 where it is warmed to a temperature of -303" F. by indirect heat exchange with the nitrogen reflux stream flowing through branch line 63. The nitrogen at a temperature of 303 F. flows through exchanger 51 where it is warmed to a temperature of 293 F. at which temperature, as hereinabove described, it enters exchanger 55.

Oxygen at a temperature of 293 F. and a pressure of 6.6 pounds flows through line 61, is warmed in its flow through exchanger 6| in indirect heat exchange with a nitrogen stream leaving column 5| through line 50 at a temperature of 287 F. From exchanger 5| the oxygen flows through line 68 into the base of column 52. From exchanger 5| a portion of the nitrogen flows through line 52 into the top of column 5|. the remainder of the nitrogen flowing through branch line 63 into exchanger 54 where it is cooled to a temperature of -30l.2 F. The nitrogen at this temperature is flashed in its flow through valve 55; its temperature is thereby reduced to about 316.5 F. and its pressure to about 5.4 pounds at which temperature and pressure it enters the top of column 52.

Upon reversal which may take place every three minutes, the air flows through paths l3 and 35 in exchangers I0 and 33, respectively, nitrogen through path l2 in exchanger NJ and oxygen through path 34 in exchanger 33. The flow of the various streams is otherwise substantially the same as hereinabove described and the temperature and pressure conditions remain the same. The nitrogen in its flow through path I2 and the oxygen in its flow through path 34 in exchangers Ill and 33, respectively, remove by sublimation and evaporation the carbon dioxide and frost, if any, deposited in these paths by the air during the preceding step. Thus in the continual operation upon each reversal the nitrogen and oxygen rectification products efiect removal of the carbon dioxide and frost, it any, deposited in the paths through which the air had passed during the preceding step of the process.

Operating in accordance with this invention it is found that substantially complete removal of carbon dioxde takes place from the air stream prior to its introduction into the rectification system permitting efficient rectification of the air to produce oxygen. Further this invention effects the purging of the air flow paths upon each reversal to remove therefrom the carbon dioxide and frost. if any, thereby permitting continuous operation. Moreover, the refrigeration necessary to compensate for cold losses resulting from the difference in enthalpy between the incoming air and the outgoing products of rectification and for heat leaks into the system is supplied by expanding a minor portion of the chilled compressed air and this without loss of the oxygen content of the expanded air.

In the operation of the process of this invention it is preferred to effect removal of moisture and carbon dioxide both in accordance with the process of this invention. It will be understood. however, that if desired th moisture may be removed from the air by any conventional means and dry air containing carbon dioxide passed through the exchanger or exchangers as hereinabove disclosed. In the event that dry air is supplied to the process and the exchanger system is similar to that of Figure 2, reversing valves l4 and 4| may advantageously be moved to the position between exchangers WA and MB and exchangers 33A and 333, respectively, 50 that reversal of the air and nitrogen streams and f the air and oxygen streams occurs only in exchangers IOB and 338, respectively, wherein the carbon dioxide is d posited by the air stream. Operation of such an arrangement is carried out so that the temperature at the warm end of exchangers IOB and 33B is at least slightly higher than the temperature at which carbon dioxide begins to deposit from the air stream. In general, the warm end of these exchangers should be at a temperature above about -180 F.

The expressions reversing the now of air and nitrogen and "reversal" are used herein in the sense commonly employed in this art, namely, to mean the switching of the flow of two streams. for example, the air and the nitrogen or oxygen streams, so that upon each reversal" the air flows through the path through which had previously flowed the nitrogen or oxygen and the nitrogen or oxygen flows through the path through which had previously flowed the air.

Since certain changes may be made in carrying out the above processes without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A process for producing oxygen by the liquefaction and rectification of air which comprises passing a stream of air through a path in a heat exchange zone, passing a stream of nitrogen rectification product through another path in said heat exchange zone in heat exchange relation with the air passing therethrough, thereby cooling the air to a temperature close to its condentation point at the pressure prevailing in said heat exchange zone and effecting substantially complete removal of carbon dioxide from the air in its passage through said heat exchange zone, warming a minor portion of the thus chilled air to a temperature such that upon subsequent expansion substantially none of the expanded air is liquefied, expanding the warmed portion of the air to produce refrigeration in amount sufficlent to compensate for cold losses resulting from the difference in enthalpy between the incoming air and the outgoing products of rectification and for heat leak; into the process, pas ing the remainder of the air at said temperature close to its condensation point to a rectification zone, maintaining the temperature difference between the temperature of the air leaving and the tem perature of the nitrogen rectification product entering said zone so that it falls within the range of about 6 to about 8' F. and periodically reversing the flow of air and nitrogen rectification product through their respective paths in said zone, whereby upon each of said reversals the nitrogen rectification product substantially completely removes the carbon dioxide deposited in said zone during the preceding step of said process.

2. The process of claim 1 wherein said temperature close to the condensation point of the air is below 270 F.

3. A process for producing oxygen by the liquefaction and rectification of air which comprises passing a stream of air through a path in a heat exchange zone, passing a stream of nitrogen rectification product through another path in said heat exchange zone in heat exchange relation with the air passing therethrough, thereby cooling the air to a temperature close to its condensation point at the pressure prevailing in said heat exchange zone and effecting substantially complete removal of carbon dioxide from the air passing through said zone, warming a minor portion of the thus chilled air by passage through said heat exchange zone, mixing said minor portion with another minor portion of the air leaving said zone to produce an air mixture having a temperature such that upon expansion substantially no liquid air is formed, passing the remainder oi the air leaving said heat exchange zone to the high pressure stage of a, rectification system. expanding the said air mixture to produce refrigeration in amount sufficient to compensate for cold losses due to the difference in enthalpy between the incoming air and the outgoing products oi rectification and for heat leaks into the process, introducing the expanded air into the low pressure stage of said rectification system, maintaining the temperature difference between the temperature of the air leaving and the temperature of the nitrogen rectification product entering said zone so that it falls within the range of about 5 to about 10 F. and periodically reversing the How of air and nitrogen through their respective paths in said zone so that upon each of said reversals the nitrogen rectification product substantially completely removes the carbon dioxide deposited in said zone during the preceding step of the process.

4. A process for producing oxygen by the liquefaction and rectification of air which comprises passing a stream of air through a path in a heat exchange zone, passing a stream of nitrogen rectification product through another path in said heat exchange zone in heat exchange relation with the air passing thcrethrough, thereby cooling the air to a temperature close to its condensation point at the pressure prevailing in said heat exchange zone and effecting substantially complete removal of carbon dioxide from the air passing through said zone, warming a minor portion of the thus chilled air by passage of at least a portion thereof through a portion only of said heat exchange zone thereby heating said minor portion to a temperature such that upon subsequent expansion substantially no liquid air is formed, passing the remainder of the air leaving said heat exchange zone to the high pressure stage of a rectification system, expanding the warmed air to produce refrigeration in amount sufficient to compensate for cold losses due to the difference in enthalpy between the incoming air and the outgoing products of rectification and for heat leaks into the process, introducing the expanded air into the low pressure stage of said rectification system, maintaining the temperature difference between the temperature of the air leaving and the temperature of the nitrogen rectification product entering said zone so that it falls within the range of about 5 to about 10 F. and periodically reversing the flow of air and nitrogen through their respective paths in said zone so that upon each of said reversals the nitrogen rectification product substantially completely removes the carbon dioxide deposited in said zone during the preceding step of the process.

5. A process for producing oxygen by the liquefaction and rectification of air which comprises passing two streams of air through two paths in a first and second heat exchange zone, passing streams of nitrogen and oxygen rectification products through other paths in said heat exchange zones in heat exchange relation with the air passing therethrough, thereby cooling the air to a temperature close to its condensation point under the pressure prevailing in said heat exchange zones and effecting substantially complete removal of carbon dioxide from the air in its passage through said zones, withdrawing a minor portion of the air leaving said zones and repassing at least a portion thereof through one of said zones to warm said minor portion to a temperature such that upon subsequent expansion substantially no liquid air is formed, introducing the warmed air into an expander, expanding the warmed air to produce refrigeration in amount suiflcient to compensate for cold losses resulting from the difference in enthalpy between the incoming air and the outgoing products 01 rectification and for heat leaks into the process, introducing the expanded air into the low pressure stage of a rectification system, passing a minor portion of the nitrogen stream leaving the low pressure stage of said rectification system through one of said zones thus warming said nitrogen, introducing this warmed nitrogen into the remaining major portion of the nitrogen to produce the said nitrogen rectification product stream passed in heat exchange relation with one of said air streams, maintaining the temperature diiierence between the temperature of the air and the temperature of the rectification product stream at the cold end of each of said zones so that it falls in the range of about 5 to about F., and periodically reversing the flow of air and nitrogen and the flow of air and oxygen through their respective paths in the said zones so that upon each of said reversals the nitrogen and oxygen substantially completely remove the carbon dioxide deposited in said zones during the preceding step 01' the process.

6. A process for producing oxygen by the liquei'action and rectification of air in a two stage rectification system involving a high and a low pressure stage which comprises passing two streams of air at a pressure of from about 60 pounds to about 100 pounds through two paths in a first and second heat exchange zone, passing streams of nitrogen and oxygen rectification products through other paths in said heat exchange zones in heat exchange relation with the air passing therethrough, thereby cooling the air to a temperature close to its condensation point at the pressure prevailing in said heat exchange zones and effecting substantially complete removal of carbon dioxide from the air in its passage through said zones, withdrawing a minor portion of the air leaving said zones, repassing at least a portion of said minor portion through one of said zones to warm said minor portion to a temperature such that upon subsequent expansion substantially no liquid air is formed, introducing the warmed air into an expander, expanding the warmed air to a pressure of from about 4 pounds to about 10 pounds to produce refrigeration in amount sufi'icient to compensate for cold losses resulting from the difference in enthalpy between incoming air and the outgoing products of rectification and for heat leaks into the process. introducing the expanded air into the low pressure stage of the rectification system, passing a minor portion of the nitrogen stream leaving the low pressure stage of said rectification system through one of said zones, thus warming said nitrogen, introducing this warmed nitrogen into the remaining major portion 01 the nitrogen withdrawn from the low pressure stage of said rectification system to produce the said nitrogen rectification product stream passed in heat exchange relation with one of said air streams in one of said heat exchange zones, maintaining the temperature difference between the temperature 0! the air and the temperature of the rectification product stream at the cold end of each of said zones so that it falls in the range of about 6 to about 8 F., and periodically reversing the flow of air and nitrogen and the fiow of air and oxygen through their respective paths in the said zones so that upon each of said reversals the nitrogen and oxygen substantially completely remove carbon dioxide deposited in said zones during the preceding step of the proces.

7. The process of claim 6 wherein said temperature close to the condensation point 01' the air is below -2'10 F.

8. A process for producing oxygen by the liquefaction and rectification oi air in a two stage rectification system involving a high and a low pressure stage which comprises passing a stream of air through a path in a first heat exchange zone, passing a stream of nitrogen rectification product through another path in heat exchange relation with the air in said first heat exchange zone, thereby cooling the air to a temperature close to its condensation point under the pressure prevailing in said heat exchange zone and effecting substantially complete removal of carbon dioxide from the air in its passage through said zone, withdrawing a minor portion of the air leaving said zone and repassing at least a portion thereof therethrough to warm said minor portion to a temperature such that upon subsequent expansion substantially no liquid air is formed, introducing the warmed air into an expander, expanding the warm air to produce reirigeration in amount sufilcient to compensate for cold losses resulting from the diflerence in enthalpy between the incoming air and the outgoing products of rectification and for heat leaks into the process, introducing the expanded air into the low pressure stage of the rectification system, passing another stream of air through a path in a second heat exchange zone, passing oxygen rectification product from the low pressure stage through another path in said second heat exchange zone in heat exchange relation with the air passing therethrough, thereby cooling the air to a temperature close to its condensation point under the pressure prevailing in said heat exchange zone and effecting a substantially complete removal of carbon dioxide from the air in its passage through said second heat exchange zone, passing a minor portion of the nitrogen stream leaving the low pressure stage of said rectifioation system through the said second mentioned heat exchange zone thus warming said nitrogen, introducing this warmed nitrogen into the remaining major portion of the nitrogen to produce said nitrogen rectification product stream passed in heat exchange relation with the air in the first mentioned heat exchange zone, maintaining the temperature diiference between the temperature of the air and the temperature of the rectification product stream at the cold end of each of said zones so that it falls in the range of about 5 to about 10 F., and periodically reversing the flow of air and nitrogen through their respectiv paths in the first mentioned zone and the fiow of air and oxygen through their respective paths in the second mentioned zone so that upon each of said reversals the nitrogen and 21 oxygen substantially completely remove the carbon dioxide deposited in said zones during the preceding step of the process.

9. A process for producing oxygen by the liquefaction and rectification of air in a two stage rectification system involving one stage maintained at a pressure of from about 60 to about 100 pounds and a low pressure stage maintained at a pressure of from about 4 to about 10 pounds, which comprises passing a stream of air at a pressure of from about 60 to about 100 pounds and a. temperature of from about 70 to about 110 F. through a path in la first heat exchange zone, passing a' stream of nitrogen rectification product through another path in said first heat exchange zone in heat exchange relation with the air passing therethrough, thereby cooling the air to a temperature below 270 F. but above its condensation point at the pressure prevailing in said first heat exchange zone and effecting substantially complete removal of carbon dioxide from the air in its passage through said first heat exchange zone, withdrawing a minor portion of the air leaving said first heat exchange zone and repassing at least a portion thereof therethrough to warm said minor portion to a temperature such that upon subsequent expansion substantially no liquid air is formed, introducing the warmed air into an expander, expanding the warmed air to a pressure of about 4 to about 10 pounds to produce refrigeration in amount sufficient to compensate for cold losses resulting from the diiference in enthalpy between incoming air and the outgoing products of rectification and for heat leaks into the process, introducing the expanded air into the low pressure stage of the rectification system, passing another stream of air at a pressure of from about 60 to about 100 pounds and a temperature of from about 10 to about 110 F. through a path in a second heat exchange zone, passing oxygen rectification product from the low pressure stage of the rectification system through another path in said second heat exchange zone in heat exchange relation with the air passing therethrough, thereby cooling the air to a temperature below 270 F. but above its condensation point at the pressure prevailing in said second heat exchange zone and effecting a substantially complete removal of carbon dioxide from the air in its passage through said second heat exchange zone, passing the major portion of the thus cooled air to the high pressure stage of the rectification system, passing a. minor portion of the nitrogen stream leaving the low pressure stage of said rectification system through the said second mentioned heat exchange zone thus warming said nitrogen, introducing this warmed nitrogen into the remaining major portion of the nitrogen to produce said nitrogen rectification product stream passed in heat exchange relation with the air in the first mentioned heat exchange zone, maintaining the temperature difference between the temperature of the air and the temperature of the rectification product stream at the cold end of each of said zones so that it falls in the range of about 5 to about F. and periodically reversing the flow of air and nitrogen through their respective paths in the first mentioned zone and the flow of air and oxygen through their respective paths in the second mentioned zone so that upon each of said reversals the nitrogen and oxygen substantially completely remove the carbon dioxide deposited in said zones during the preceding step of the process.

10. A process for producing oxygen by the liquefaction and rectification of air in a two stage rectification system involving a high and a low pressure stage which comprises passing a stream of air through a path in a heat exchange zone, passing a stream of oxygen rectification product through another path in said heat exchange zone in heat exchange relation with the air passing therethrough, passing a minor portion of the nitrogen rectification product through said heat exchange zone, thereby warming said minor portion, mixing the warmed minor portion with the remaining major portion of the nitrogen product of rectification and introducing the resulting mixture at a temperature of from about 5' to about 10 F. below the temperature of the air leaving said zone into still another path in said heat exchange zone and passing it therethrough in heat exchange relation with the other streams passing through said heat exchange zone, withdrawing a minor portion of the air stream leaving said heat exchange zone and repassing at least a portion thereof through said heat exchange zone to warm the said minor portion of the air to a temperature such that upon subsequent expansion substantially no liquid air is formed, thereby cooling the air stream leaving said heat exchange zone to a temperature close to its condensation point at the pressure prevailing in said heat exchange zone and effecting substantially complete removal of carbon dioxide from said air stream, expanding the warmed air to produce refrigeration in amount suflicient to compensate for cold losses resulting from the difference in enthalpy between the incoming air and the outgoing products of rectification and i'or'heat leaks into the process, introducing the expanded air into the low pressure stage of the rectification system, passing the major portion of the air leaving said heat exchange zone to the high pressure stage of said rectification system and periodically reversing the flow of air and nitrogen through their respective paths in said zone so that upon each of said reversals the nitrogen substantially completely removes the carbon dioxide deposited in said zone during the preceding step of the process.

11. A process for producing oxygen by the liquefaction and rectification of air in a two stage rectification system involving a high and a low pressure stage which comprises passing a stream of air through a path in a heat exchange zone, passing from the low pressure stage of the rectification system a stream of oxygen through another path in said heat exchange zone in heat exchange relation with the air passing therethrough, withdrawing a minor portion of the air stream leaving said heat exchange zone and repassing it through said heat exchange zone to warm the said minor portion of the air, passing the thus warmed air in indirect heat exchange relation with a minor portion of the nitrogen withdrawn from the low pressure stage of the rectification system thereby warming this minor portion of the nitrogen and cooling said minor portion of the air to a temperature such that upon subsequent expansion substantially none of the air is liquefied, mixing the warmed minor portion of the nitrogen with the remaining major portion of the nitrogen withdrawn from the low pressure stage of the rectification system to produce a mixture having a temperature of from about 5 to about 10 F. below the temperature of the air leaving said zone, passing the resulting mixture through still another path in said heat exchange zone in heat exchange relation with the other streams passing through said heat exchange zone. thereby cooling the air stream leaving said heat exchange zone to a temperature close to its condensation point at the pressure prevailing in said heat exchange zone and effecting substantially complete removal of carbon dioxide from said air stream, expanding at least the said cooled minor portion of the air to produce refrigeration in amount sufficient to compensate for cold losses resulting from the diil'erence in enthalpy between the incoming air and the outgoing products of rectification and for heat leaks into the process, introducing the expanded air into the low pressure stage of the rectification system, passing the major portion of the air leaving said heat exchange zone to the high pressure stage of said rectification system and periodically reversing the flow of air and nitrogen through their respective paths in said zone so that upon each of said reversals the nitrogen substantially completely removes the carbon dioxide deposited in said zone during the preceding step of the process.

12. A process for producing o ygen by the liquefaction and rectification of air in a two stage rectification system involving a high and a low pressure stage which comprises passing a stream of air through a path in a heat exchange zone, passing from the low pressure stage of the rectitlcation system a stream of oxygen through another path in said heat exchange zone in heat exchange relation with the air passing therethrough, withdrawing a minor portion of the air stream leaving said heat exchange zone, withdrawing a minor portion of the nitrogen stream leaving the low pressure stage of said rectification system, warming one of said minor streams by passage through said heat exchange zone, passing the thus warmed stream in heat exchange relation with the other minor stream to warm it thereby producing warmed minor streams of air and nitrogen, said warmed minor air stream being at a temperature such that upon subsequent expansion substantially no liquid air is formed, mixing the warmed minor nitrogen stream with the remaining major portion of the nitrogen withdrawn from the low pressure stage of the rectification system to produce a nitrogen stream at a temperature of from about to about F. below the temperature of the air leaving said zone, passing the resulting nitrogen stream through still another path in said heat exchange zone in heat exchange relation with the other streams passing through said heat exchange zone, thereby cooling the air leaving said zone to a temperature close to its condensation point at the pressure prevailing in said zone and effecting substantially complete removal of carbon dioxide trom said air stream, expanding at least the warmed minor air stream to produce refrigeration in amount sufllcient to compensate for cold losses resulting from diil'erence in enthalpy between the incoming air and the outgoing products of rectification and for heat leaks into the process, introducing the expanded air into the low pressure stage of the rectification system, passing the major portion of the air leaving said heat exchange zone to the high pressure stage of said rectification system and periodically reversing the flow of air and nitrogen through their respective paths in said zone so that upon each of said reversals the nitrogen substantially completely removes the carbon di- 24 oxide deposited in said zone during the preceding step of the process.

13. A process for producing oxygen by the liquefaction and rectification of air, which comprises passing a stream of rectification product through a path in a heat exchange zone, passing a stream of air through another path in said heat exchange zone to recover the cold content of the rectification product stream and thereby cool the air to a temperature close to its condensation point at the pressure prevailing in said heat exchange zone and effect substantially complete removal of carbon dioxide from the air in its passage through said heat exchange zone, warming a portion of the thus chilled air to a temperature such that upon subsequent expansion substantially none of the expanded air is liquefied, expanding the warmed portion of the air to produce refrigeration in amount suflicient to compensate for cold losses resulting from the difference in enthalpy between the incoming air and the outgoing products of rectification and for heat leaks into the process, passing the remainder of the air at said temperature close to its condensation point to a rectification zone, maintaining the temperature difference between the temperature of the air leaving and the temperature of the rectification product entering said zone so that it falls within the range of about 5 to about 10 F. and periodically reversing the flow of air and rectification product through their respective paths in said zone, whereby upon each of said reversals the rectification product substantially completely removes the carbon dioxide deposited in said zone during the preceding step of the process.

14. The process of claim 13 wherein said stream of rectification product passed in heat exchange relation with the air is a nitrogen stream and said temperature close to the condensation point of the air is below 2'70 F.

15. The process of claim 13 wherein said warming of a portion of the thus chilled air is effected by passing a part Of said portion through still another path in said heat exchange zone and then mixing said part with the remaining part prior to expanding the resulting warmed portion of the air.

16. A process for producing oxygen by the liquefaction and rectification of air, which comprises passing a stream of nitrogen rectification product through a path in a heat exchange zone, passing a stream of air through another path in said heat exchange zone to recover the cold content of the nitrogen rectification product and thereby cool the air to a temperature close to its condensation point at the pressure prevailing in said heat exchange zone and effect substantially complete removal of carbon dioxide from the air passing through said zone, warming a minor portion of the thus chilled air by passage of at least a portion thereof through said heat exchange zone thereby heating said minor portion to a temperature such that upon subsequent expansion substantially no liquid air is formed, expanding the warmed air to produce refrigeration in amount sufficient to compensate for cold losses resulting from the difference in enthalpy between the incoming air and the outgoing prodnets of rectification and for heat leaks into the process, introducing the expanded air into the low pressure stage of a rectification system, maintaining the temperature difference between the temperature of the air leaving and the temperature of the nitrogen rectification product 25 26 entering said zone so that it falls within the range of about 5 to about 10 F. and periodically E ER NCES CITED reversing the now of 3111' and nitrogen through The following references are of record in the their respective paths in said zone so that upon fil f this t each of said reversals the rectification product 5 substantially completely removes the carbon di-"' UNITED STATES PATENTS oxide deposit-ed in said zone during the preced- Number Name Date ins step 01' the process. 2,460,859 Trumpler Feb. 8. 1949 FRANK J JENNY. 10 

1.A PROCESS FOR PRODUCING OXYGEN BY THE LIQUEFACTION AND RECTIFICATION OF AIR WHICH COMPRISES PASSING A STREAM OF AIR THROUGH A PATH IN A HEAT EXCANGE ZONE, PASSING A STREAM OF NITROGEN RECTIFICATION PRODUCT THROUGH ANOTHER PATH IN SAID HEAT EXCHANGE ZONE IN HEAT EXCHANGE RELATION WITH THE AIR PASSING THERETHROUGH, THEREBY, COOLING THE AIR TO A TEMPERATURE CLOSE TO ITS CONDENSATION POINT AT A TEMPERATURE PREVAILING IN SAID HEAT EXCHANGE ZONE AND EFFECTING SUBSTANTIALLY COMPLETE REMOVAL OF CARBON DIOXIDE FROM THE AIR IN ITS PASSAGE THROUGH SAID HEAT EXCHANGE ZONE, WARNING A MINOR PORTION OF THE THUS CHILLED AIR TO A TEMPERATURE SUCH THAT UPON SUBSEQUENT EXPANSION SUBSTANTIALLY NONE OF THE EXPANDED AIR IS LIQUIFIED, EXPANDING THE WARMED PORTION OF THE AIR TO PRODUCE REFRIGERATION IN AMOUNT SUFFICIENT TO COMPENSATE FOR COLD LOSSES RESULTING FROM THE DIFFERENCE IN ENTHALPY BETWEEN THE INCOMING AIR AND THE OUTGOING PRODUCTS OF RECTIFICATION AND FOR HEAT LEAKS INTO THE PROCESS, PASSING THE REMAINDER OF THE AIR AT SAID TEMPERATURE CLOSE T ITS CONDENSATION POINT TO A RECTIFICATION ZONE, MAINTAINING THE TEMPERATURE DIFFERENCE BETWEEN THE TEMPERATURE OF THE AIR LEAVING AND THE TEMPERATURE OF THE NITROGEN RECTIFICATION PRODUCT ENTERING SAID ZONE SO THAT IT FALLS WITHIN THE RANGE OF ABOUT 6* TO ABOUT 8* F. AND PERIODICALLY REVERSING THE FLOW OF AIR AND NIREOGEN RECTIFICATION PRODUCT THROUGH THEIR RESPECTIVE PATHS IN SAID ZONE, WHEREBY UPON EACH OF SAID REVERSALS THE NITROGEN RECTIFICATION PRODUCT SUBSTANTIALLY COMPLETELY REMOVES THE CARBON DIOXIDE DEPOSITED IN SAID ZONE DURING THE PRECEDING STEP OF SAID PROCESS. 